1. Field of the Invention
This invention relates to a synchronizing signal selecting circuit which works to select proper synchronizing signals from television video signals, and more particularly to a synchronizing signal selecting circuit which is most suitable for a synchronizing signal generator.
2. Description of the Prior Art
FIG. 1 shows a well known synchronizing signal generator by which synchronizing signals are generated with reference to the synchronizing signals included in the television video signals. Transmitted video signals are supplied to an input terminal 1, and led to a burst separating circuit 2 and a synchronizing separtor circuit 3. A burst signal is separated in the burst separating circuit 2. A horizontal synchronizing signal a and a vertical synchronizing signal v are separated in the synchronizing separator circuit 3. The burst signal from the burst separating circuit 2 is averaged and controlled into a predetermined frequency (for example, 14.3 MHz) in a synchronizing AFC circuit 4. The output of the synchronizing AFC circuit 4 is supplied to a frequency divider 5 which includes a horizontal counter and a vertical counter. A horizontal synchronizing signal is obtained from the horizontal counter by which the output of the synchronizing AFC circuit 4 is divided, in the frequency divider 5. The frequency of the obtained horizontal synchronizing signal is divided by the vertical counter to generate a vertical synchronizing signal in the frequency divider 5.
In the operation of the frequency divider 5, the horizontal synchronizing signal a and the vertical synchronizing signal v are respectively supplied from the synchronizing separator circuit 3 to the frequency divider 5 to reset the horizontal counter and vertical counter of the frequency divider 5. With such reset operation, the new synchronizing signals obtained from the frequency divider 5 are synchronously combined with the synchronizing signals included in the video signals applied to the input terminal 1 (gen-lock).
In the well known synchronizing signal generator, noises included in the output of the synchronizing separator circuit 3 can similarly reset the horizontal counter and vertical counter of the frequency divider 5, to disturb the synchronous combination. Accordingly, it is required for the exact synchronous combination that only proper horizontal and vertical synchronizing signals are supplied to the frequency divider 5, even though they are not periodically supplied to the frequency divider 5.